1. Field of the Invention
The present invention relates to a system and a method of controlling a fluid temperature for improving the injectivity of supercritical carbon dioxide into a stratum.
2. Description of the Related Art
A geological storage technology of carbon dioxide (CO2) is to geologically inject the CO2, which is exhausted into the atmosphere, to isolate the CO2 from the atmosphere. The geological storage technology has been already proven and is in the commercialization step in some nations. In general, if the storage place of the CO2 is selected, an injection well system is installed to geologically store the CO2. First, if a bore is formed to a target depth (about 800 m) for injection under the ground, in which CO2 can exist in the supercritical state, by using a boring machine and an injection well is formed, a casing is installed in the injection well, and cement is grouted throughout the whole peripheral space between the casing and a counterforted wall, thereby preventing the leakage path of the injected CO2 to the ground surface. Thereafter, several small holes are formed by perforating the casing and the cement corresponding to a target section for CO2 injection by using a perforating gun, thereby forming a connection passage between the injection well and the peripheral stratum. The supercritical CO2 injected through the injection well is injected into the stratum through the connection passage, so that the geological storage of the CO2 is achieved. In the geological storage of the CO2, the injectivity, the storage capacity, and the integrity in which the injected CO2 does not leak to the ground surface must be mainly taken into consideration.
If the CO2 injection system is completed, the temperature and the pressure suitable for the geological storage of the CO2 are determined at the upper portion of the injection well, and the CO2 is injected. However, the temperature and the pressure of the CO2 when the CO2 is injected may be changed according to the geothermal gradient and the hydrostatic pressure gradient as the CO2 is transferred down to the storage depth of the injection well through a stratum.
In general, the pressure at the injection depth is determined by following equation.PIP=PWH+g∫0zρ(z)dz  Equation
In the equation, PIP refers to a pressure at the injection depth of the lower portion of the injection well, PWH refers to a pressure at the upper portion of the injection well, g refers to gravitational acceleration, and ρ(z) refers to the density of the CO2 at the injection depth z of the injection well.
In other words, the density of the CO2 varies according to the temperature and pressure under the ground, so that the variation in the density of the CO2 affects an injection pressure at the injection depth. The CO2 density in the storage place affects the mobility of the CO2, so that an influence is exerted on the two-phase flow of the CO2 in the air gaps filled with salt water.
If the supercritical CO2 is injected by using a conventional injection well system, the temperature and the pressure of the CO2 may be adjusted at the upper portion of the injection well. However, the temperature and the pressure of the injected CO2 varying as the CO2 passes through the stratum cannot be adjusted, so that the CO2 cannot be effectively injected.
In general, the injectivity of the CO2 is determined by following equation.
                    Injectivity        =                  Q                                    P                              bh                ⁢                                                                  ⁢                p                                      -                          P              res                                                  Equation      
In this equation, Q refers to an amount of injected CO2, Pbhp refers to a pressure at the injection depth of the lower portion of the injection well, and Pres refers to a pressure at the target stratum for CO2 injection. In other words, this equation refers to that the injectivity is increased if the pressure is reduced at the injection depth of the lower portion of the injection well on the assumption that the amount of the injected CO2 is constant
If the injectivity is reduced, many injection wells are required, so that economic loss may be caused. In addition, if the pressure in the target stratum for the storage of the CO2 is increased, the stress is changed in the stratum around the injection well, so that the stratum can be finely cracked. Accordingly, the injected CO2 may be leaked to the ground surface through the crack. In other words, the excess pressure increase caused by the injection of the CO2 exerts bad influences on both the injectivity and the leakage of the CO2. Accordingly, a scheme to prevent the increase of the pressure in the target stratum for the CO2 injection is required
In general, the geological storage of the CO2 is achieved by injecting the CO2 into a sand stone layer saturated with salt water that cannot be used as drinking water. If the supercritical CO2 is injected into a stratum saturated with salt water, the supercritical CO2 dispels the salt water so that the supercritical CO2 is stored in the air gap. In this case, a small amount of salt water remains in the air gaps by a residual saturation value. The water of the remaining salt water is evaporated by the supercritical CO2, so that the air gaps are fully saturated with the supercritical CO2, thereby forming a dry-out zone. If water of remaining salt water in the air gaps of the dry-out zone is evaporated by the supercritical CO2, salt is deposited. The deposited slat is filled in the air gaps, so that the porosity and the transmissivity are reduced. Accordingly, the pressure is increased, so that the injectivity may be reduced.